Revert "Stop readahead thread if the file isn’t opened."

IsOpen() attempts to lock our QMutex; which causes a deadlock.

This reverts commit 6b9b113b3e83e4298fb4b62ed170b9d94d9b2007.

(cherry picked from commit 3a6d226e2c40b949a70ea1e9383a0e11b5e100c0)
Stop readahead thread if the file isn’t opened try #2

No point trying to read from the file when there’s nothing to read

(cherry picked from commit 338c49a7356a8a4637dfe2047d40b14cb3960393)